The present invention relates generally to the field of optical signal amplifiers and in particular, to fluorophosphate glass compositions for use in optical signal amplifiers operating at wavelengths around 1.55 xcexcm.
Optical signal amplifiers have quickly found use in optical telecommunication networks, particularly in those networks using optical fiber over long distances. Although modem silica-based optical fibers general exhibit relatively low loss in the 1.55 xcexcm window, they are lossy to some extent and the loss accumulates over distance. To reduce this attenuation, opto-electronic devices have been used to boost signal power. These devices require that the optical signal be converted to an electronic signal. The electronic signal is then amplified using commonly known amplification techniques and is reconverted back to an optical signal for re-transmission.
Optical signal amplifiers amplify optical signals without requiring an opto-electronic conversion of the signal. In optical amplifiers, the weakened light signal is directed through a section of an amplifying medium that has been doped with ions from a rare earth element. Light from an external light source, typically a semi-conductor laser, is pumped into the amplifying medium stimulating the rare earth atoms to a higher energy level. Light entering the amplifying medium at the signal wavelength further stimulates the excited rare earth ions to emit their excess photon energy as light at the signal wavelength in phase with the signal pulses, thereby amplifying the light signal. One type of optical amplifier uses a length of erbium-doped optical fiber. Erbium-doped fiber amplifiers (EDFA) are usually doped on the order of 100-500 ppm of erbium ion. Typical EDFA fiber lengths are on the order of 10-30 meters, depending on the final gain requirements necessary for a particular application. In some applications, it is impractical to use a 10-30 meter length of fiber. Planar-type optical amplifiers have been developed for use in more confined spaces. The useful length of a planar amplifying device is generally no more than 10 centimeters. To achieve the same amplification levels as a 10 to 30 meter length EDFA, a planar amplifier requires an amplifying medium with a higher concentration of erbium ions, on the order of up to 4 to 7 percent by weight.
However, in known types of optical amplifying medium, several gain loss mechanisms occur at high erbium ion concentration levels, including ion clustering and cooperative homogenous upconversion (concentration quenching). Because erbium ions do not dissolve well in a silica matrix, erbium ions will cluster, allowing energy transfer in the clustered region. In addition, at higher erbium concentrations, ion-to-ion interaction becomes more significant. The resulting energy upconversion quenches the inverted population. Erbium ion energy is used in the clustering and quenching processes and is therefore unavailable for the required amplifying phonon process. As a result, quantum efficiency of the amplifying medium decreases rapidly with higher erbium ion concentration, with a concomitant decrease in amplifier gain.
Yet further, known silica-based erbium-doped amplifiers exhibit a distinct spectral nonuniformity of gain. The lack of a flat gain spectrum over a wide bandwidth causes several problems. For instance, extremely short optical pulses have a relatively wide power spectrum and are not accurately amplified if the gain spectrum is not flat. In addition, in larger bandwidth applications, such as wavelength division multiplexing (WDM), the fiber receives data-modulated optical signals from several optical transmitters, each using a different optical carrier frequency. If the gain spectrum from the optical amplifier is not flat over the operating wavelength, the carrier frequencies at gain peaks might saturate while the carrier frequencies at the skirts and valleys may not be sufficiently amplified. Past efforts to address gain flattening have primarily relied on passive or active filtering of the high gain features of the gain spectrum. However, this requires a close matching of the particular amplifier and filter and must account for temporal variations in the gain spectrum.
The present invention is concerned with a family of glasses that find particular utility in production of optical signal amplifiers. These glasses are doped with high concentrations (up to 10 wt. %) of erbium oxide while exhibiting weak concentration quenching behavior. These glasses also provide higher fluorescence efficiency an more uniform gain characteristics than known silicate and fluorozirconate glass medium. These glasses provide high and flat gain characteristics that are particularly useful for optical amplification in the 1.55 xcexcm optical wavelength window, and are particularly well suited for use in wavelength division multiplexing (WDM) systems
One aspect of the present invention is directed to a family of glasses, particularly fluorophosphate glasses, that are particularly well suited for high rare earth ion concentration levels. It is an object of the present invention to provide a fluorophosphate glass medium doped with erbium oxide ions for use in an optical amplifier for providing flat and high gain in an optical wavelength window around 1.55 xcexcm. The fluorophosphate glasses of the present invention comprise high concentrations of erbium ions (i.e. close to 10% by weight) and provide a more spectrally uniform gain, similar to ZBLAN  less than xe2x88x92 greater than , and significantly improved over typical silicate and phosphate glass compositions.
The present invention is directed to a family of glass for optical amplification comprising a substantially silica free fluorophosphate glass medium, doped for 100 parts by weight constituted of:
with up to 10, preferably between 0.01 and 10, parts by weight of erbium oxide.
Preferably, the fluorophosphate glass according to the present invention has a composition, comprising in parts by weight:
The fluorophosphate glass according to the present invention may also be co-doped with up to 15 parts by weight of Yb2O3 as a sensitizer to increase pump efficiency at around 980 nm. The fluorophosphate glass according to the invention preferably has an index of refraction between about 1.48 and 1.58.
According to another aspect, the present invention is directed to an erbium-doped optical amplifier for a wavelength band of approximately 1.55 xcexcm, having a medium for optical amplification comprising a substantially silica free fluorophosphate glass composition that further comprises in addition to 100 parts by weight of other components, about 0.01 to 10 parts by weight of Er2O3. The optical amplifier according to the present invention may be either a planar-type optical amplifier or a single mode fiber type optical amplifier.
The optical amplifier according to the present invention includes fluorophosphate glass comprising, for 100 parts by weight constituted by:
up to 10, and preferably between 0.01 and 10 parts by weight of erbium oxide.
The fluorophosphate glass used for the optical amplifier according to the present invention may also be doped with up to 15 parts by weight of Yb2O3 as a sensitizer to increase pump efficiency at around 980 nm, and preferably has an index of refraction between about 1.48 and 1.58. Optical amplifiers according to the present invention are particularly useful in wavelength division multiplexing (WDM) systems.